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javamultithreadingconcurrencyexecutorservicejava.util.concurrent
Wudong
Wudong

Reputation: 2360

Java's ExecutorService performance

I have a main thread which dispatches jobs to a thread pool. I'm using Java's Executor framework.

From the profiler (VirtualVM) I can see each thread's activity: I can see that the main thread is waiting a lot (because the executor's queue has a upper limit) which means the executor's queue is full most of the time. However the executor's threads are not as busy as I would have thought. Most of them have a waiting time of 75%. In virtualVM it says it waits on Monitor.

Can anyone explain why is this happenning? why would the executor threads wait while there is still plenty of work available to do? And how to improve the performance of the executor? thus to improve the performance overall? More detail on the executor's wait on monitor would be great.

The job runs in the workers is just some computation, which don't depends on anything else and don't communicate to any other thread (no synchronisation), except in the end, it put data in the database, using it is own connection.

Upvotes: 3

Views: 2283

Answers (4)

ltebean
ltebean

Reputation: 1019

Thread is scheduled by the scheduler to assign cpu cycles to run them, that means if the machine has 4 cpus, at a time only 4 threads could be run in parallel, so other threads have to wait for the scheduler to assign cpu to them.

Upvotes: 0

Wudong
Wudong

Reputation: 2360

After doing a thread dump, it turns out that it is the database layer that has the synchronisation. Hibernate's Sequence Generator is synchronised.

"pool-2-thread-1" - Thread t@13
   java.lang.Thread.State: BLOCKED
    at org.hibernate.id.SequenceHiLoGenerator.generate(SequenceHiLoGenerator.java:73)
    - waiting to lock <61fcb35> (a org.hibernate.id.SequenceHiLoGenerator) owned by "pool-2-thread-5" t@23
    at org.hibernate.internal.StatelessSessionImpl.insert(StatelessSessionImpl.java:117)
    at org.hibernate.internal.StatelessSessionImpl.insert(StatelessSessionImpl.java:110)
    at ac.uk.ebi.kraken.unisave.storage.impl.HibernateStorageEngine.saveEntryIndex(HibernateStorageEngine.java:269)
    at ac.uk.ebi.kraken.unisave.storage.impl.EntryStoreImpl.storeEntryIndex(EntryStoreImpl.java:302)
    at ac.uk.ebi.kraken.unisave.impl.MTEntryIndexLoader$EntryIndexLoader.run(MTEntryIndexLoader.java:129)
    at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:471)
    at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:334)
    at java.util.concurrent.FutureTask.run(FutureTask.java:166)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1110)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:603)
    at java.lang.Thread.run(Thread.java:679)

   Locked ownable synchronizers:
    - locked <3d360c93> (a java.util.concurrent.ThreadPoolExecutor$Worker)

Upvotes: 0

Dariusz
Dariusz

Reputation: 22291

Parallel execution will yield significantly better results that a synchronous execution if:

  1. the work to be done is independent from each other (no or few and very short critical sections)

  2. each single executed work takes enough time to make up for thread start / executor's internal synchronization

  3. the work does not use the same resource - for example reading multiple files from the same disk will probably be slower than reading them sequentially.

  4. you actually have enough system resources (processor cores, memory, network speed) to use at once

Upvotes: 3

Juned Ahsan
Juned Ahsan

Reputation: 68715

Threading does not mean that all the threads will work in parallel all the time. Threads will surely go to waiting state due to various reasons, mostly depend on how the scheduler assigns the CPU to each of them. Is there some synchronized code in your thread class? If yes then if one thread is executing a synchronized method then all the other threads have to wait. If there is too much of synchronized code then threads waiting time will increase.

Upvotes: 1

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